This invention relates to apparatus and methods for improved operation of a distillation column. The invention also relates to an improved method for addressing the problem of entrainment of liquid droplets by a rising vapor stream within the flash zone of a distillation column.
Separation units such as distillation units and product strippers are major processing units in a refinery. Fractional distillation is used commercially to separate or purify fluids, and is usually carried out in a distillation column filled with either packing or trays. A trayed column consists of a stack of trays inside a shell. The trays are provided with holes to permit vapor to rise and vertical channels (downcomers) to permit the liquid to descend. Liquid and vapor are mixed on each tray, thus affecting a stage of distillation. For some columns, more than one downcomer per tray is used to reduce the liquid gradient and to provide liquid handling capacity on the tray. Trays of this type are often referred to as “multi-pass” trays. Trays of this type are described, for example, in U.S. Pat. No. 4,995,946, the entire disclosure of which is incorporated herein by reference for all purposes. Each tray is typically designed to ensure that the liquid and vapor are mixed completely and in the right proportion and that none of the liquid or vapor is permitted to leave a tray without this mixing. This is particularly important for multi-pass trays. Fractional distillation may be accomplished at sub-atmospheric pressure, at atmospheric pressure and at super-atmospheric pressure.
U.S. Pat. No. 4,698,138 teaches a chimney comprising a riser and hat which has an annulus between them. This patent teaches that the cross-sectional area of the annulus is less than that of the cross-sectional area of the riser, so that the momentum imparted to the entrained droplets drives them downwardly while the lighter vapor undergoes the second direction change and continues upward.
U.S. Pat. No. 4,714,542 teaches a process involving the introduction of a vaporizing liquid feed into a mixing and separation chamber of a flash zone via a tangential nozzle, which directs the feed into a circumferential path to enhance mixing, and the redirection of rising vapors from the distillation below the flash zone by baffling these vapors into the chamber inlet.
U.S. Pat. No. 5,972,171 teaches a de-entrainment tray that has a tray deck and a plurality of risers extending vertically up from the tray deck. The risers are provided with devices for imparting rotational movement to the fluid stream, entering the riser. The rotational movement imparted to the fluid stream causes the liquid to separate from the vapor stream and to flow upward along the riser walls. The de-entrained liquid is then transported from above the separation tray back into the flash zone by way of a liquid downcomer. The vapor stream separates from the liquid within the riser and flows out of the riser upward through the column. A riser hat is used to prevent a wash liquid from entering the riser and preferably also from entering the liquid downcomer.
The feed stream to a vacuum distillation unit is generally under turbulent conditions and thus the feed is easily entrained in the vapors that are being flashed off from the incoming feed stream. Entrainment is undesirable because heavier liquid hydrocarbons are carried upward into the distillation column and recovered in lower boiling product fractions, reducing the separation efficiency and broadening the boiling range of the individual recovered fractions. When heavier fractions in a distillation column feed contain metals, entrainment causes lighter, clean distillate fractions to be contaminated with metals. Metal-containing streams can cause severe fouling problems to pipes, pumps, valves and catalytic processes downstream of the distillation column. The reduced fractionation efficiency resulting from entrainment increases as the liquid load in the distillation column increases. Increased liquid load is particularly troublesome in the stripping section of the distillation column below the feed section. In order to meet these challenges, multipass trays may be installed in the stripping section to increase efficiency. But, operating these multi-pass trays at high efficiency requires that liquid passed to these trays from the flash zone be done in a manner to take advantage of the operation of these trays. Additional innovation is required to address the matter of passing liquids from the flash zone to the stripping section.